1. Field of the Invention
The present invention relates to a method for producing carbon fiber bundles.
2. Description of the Related Art
Carbon fiber bundles are usually produced by carbonization as follows: acrylic fiber bundles as precursors for the carbon fiber bundles are subjected to a so-called flame-retarding treatment in which the acrylic fiber bundles are passed through an oxidizing atmosphere oven (hereinafter, referred to as a flame-retarding oven) set at from 200 to 300° C.; then the flame-retardant fiber bundles are sequentially passed for carbonization through an inert atmosphere furnace (hereinafter, referred to as an precarbonization furnace) with the highest treatment temperature of from 500 to 800° C. and another inert atmosphere furnace (hereinafter, referred to as a carbonization furnace) with the highest treatment temperature exceeding 1000° C. Moreover, where necessary, by passing for graphitization the carbon fiber bundles through an inert atmosphere furnace (hereinafter, referred to as a graphitization furnace) in which the highest treatment temperature exceeds 2000° C., high-elasticity graphitized fiber bundles can be produced.
In the flame-retarding oven, the precursor fiber bundles are heat-treated in the oxidizing atmosphere, and hence the precursor fiber bundles undergo oxidation reaction to generate heat. A heat treatment temperature of the flame-retarding oven is set at as low as 200 to 300° C., lest the heat of the reaction should be stored inside the fiber bundles to take fire, and hence a long time heat treatment is required for the purpose of obtaining predetermined flame-retardant fiber bundles.
In the case where a demand for carbon fibers is increased and the production amount is intended to be increased, a multitude of fiber bundles are simultaneously fed to the oven or a baking rate is increased. However, for the purpose of increasing a production capacity by simultaneously feeding a multitude of fiber bundles, a long time treatment at a lower temperature is required, lest the heat of the reaction should be stored inside the fiber bundles to take fire, and hence such a method as simultaneously treating a multitude of fiber bundles has its own limits. An increase of the production capacity due to an increase of the baking rate may be attained by increasing the length of the precursor fiber bundles traveling in the flame-retarding oven. For the purpose of increasing the length of the precursor fiber bundles traveling in the flame-retarding oven, there is usually adopted a method in which the precursor fiber bundles are once allowed to go to outside the flame-retarding oven, and then are repeatedly passed through the flame-retarding oven in a manner turned over by turn-over rolls disposed outside the flame-retarding oven.
The flame-retardant fiber bundles completed in the heat treatment in the flame-retarding oven are treated in the precarbonization furnace, filled with an inert gas atmosphere so as for the fiber bundles not to be oxidized, with the highest treatment temperature of from 500 to 800° C., then continuously passed through the carbonization furnace in which in an inert gas atmosphere filled therein, the precarbonized fiber bundles are treated with the highest treatment temperature exceeding 1000° C., and thus converted into carbon fiber bundles. The fiber bundles being converted into carbon fiber bundles are extremely weak, a partial breakage occurs in the fiber bundles to generate fluff of the fiber bundles, in an extreme case the fiber bundles themselves are cut, and hence traveling of the fiber bundles is required to be carefully performed. Additionally, in this process, the heat treatment is usually completed in one pass because of the following and other reasons: the conversion into carbon fiber bundles occurs in an extremely short time; the temperature increase rate of the fiber bundles significantly affects the quality of the carbon fiber bundles; decomposed products occur in large amounts in the stage of conversion into the carbon fiber bundles and hence the repeated passage of the fiber bundles through the inside of the furnace contaminates the fiber bundles with such decomposed products to offer the causes to degrade the quality of the fiber bundles. In the case where the demand for carbon fibers is increased and the production amount is intended to be increased, the baking rate is increased or a multitude of fiber bundles are simultaneously fed to the furnace. The increase of the production capacity based on the increase of the baking rate leads to the extension of the furnace length and such extension is limited, and hence a multitude of fiber bundles may be simultaneously fed to the furnace.
Patent Literature 1 discloses a method for producing with satisfactory productivity carbon fibers having good quality by decreasing tow width according to a density increase of acrylonitrile-based precursor fibers. In this method, however, the traveling pitch of the precursor fibers is sometimes decreased in the flame-retarding step, and hence the heat storage due to the heat of reaction, inside the fiber bundles, sometimes cannot be removed.
Accordingly, such a method that the treatment temperature is increased, as usually performed, in the flame-retarding step according to the density increase of the precursor fibers sometimes cannot be performed, and thus the flame-retarding treatment sometimes takes a long time, as a result, the productivity is sometimes rather degraded.
Patent Literature 2 discloses a method in which heat efficiency is increased as follows: a multitude of flame-retardant fiber bundles discharged from the flame-retarding oven are divided into a plurality of groups of fiber bundles, each of the groups are brought closer to each other with respect to the horizontal direction and each of the groups forms a tier with respect to the vertical direction, accordingly the shape of the inlet of the carbonization furnace, for feeding the flame-retardant fiber bundles, is not made flat in shape, and thus the heat efficiency is increased. In this method, however, the heating conditions are sometimes vertically varied among the groups of fiber bundles, vertically divided into a plurality of stages, and accordingly the physical properties of the carbon fiber bundles may be varied among the carbon fiber bundle groups and the quality of the carbon fiber bundles may be unstable.